1. Field of the Invention
The present invention relates to the cooling of electronic circuitry, and in particular to the cooling of electronic circuitry comprising a circuit board and one or more electronic components that generate heat in use and that are mounted on the circuit board.
2. Description of the Related Art
As the functionality and performance of semiconductor integrated circuits (ICs) increase, so does the amount of dissipated heat per unit of their surface area. To control the rising temperatures of components on Printed Circuit Boards, improvement in cooling performance is required.
Most mainstream electronics cooling methods rely on convection, conduction or a combination of these (radiation only plays a negligible role in cooling electronics).
A known method of cooling ICs is through forced convective cooling. However, such forced convective cooling is constrained by the fact that the generated heat has to travel from the junction of heat dissipating components through a series of thermal resistances before it finally reaches the coolant medium, at the cost of a significant temperature gradient. These thermal resistances consist of, for instance, adhesive layers, encapsulation resins, solder connections, pockets/layers of stationary air, etc.
In another known method, high-performance ICs in consumer electronics such as Central Processing Units (CPUs) and Video Controllers dissipate most of their heat through their top surface to a heat sink or a more sophisticated (active) cooling device, with minor impact on the PCB design itself. This technique has its limitations as in most cases IC package thermal resistance from the semiconductor die to the package top surface is relatively high. In the struggle to keep pace with increasing semiconductor performances, both the size and power consumption of such add-on cooling devices have, disadvantageously become increasingly large.
Moreover, in some applications the component's top surface is not even accessible, in particular when the component is an integrated sensor. In these cases, most of the heat must be dissipated through the bottom of the component into the Printed Circuit Board and initially removed through conduction. Although in many cases the bottom side of the electronic components has the lowest thermal resistance, this side cannot be directly exposed to a coolant medium because it is facing the PCB. Generally, PCBs have a poor thermal conductivity, which can be moderately improved, e.g. by adding more or thicker copper layers.
Variants of the prior art methods mentioned above have focused on improving heat transfer by reducing the magnitude of thermal resistances, for instance by improving thermal conductivity or improving heat transfer to a coolant medium. Strictly speaking, the thermal path remains essentially the same; the improvement is merely due to improvement of sub-optimal thermal resistances. The result of these approaches is a moderately reduced temperature gradient, but at the expense of more costly material compositions or more complex, additional hardware, or both. With increasing heat dissipation levels, the amount of add-on hardware and materials becomes an increasingly dominant factor in the design, and consequently also a significant cost factor.
Another known method of cooling integrated circuits relates specifically to the cooling of printed circuit board (PCB) units contained within a main housing, in which each PCB unit is, in turn, contained within its own housing within the main housing. The method is described in GB 2 382 932 and provides for the use of relatively large apertures in a PCB and its housing for passage of forced cooling air from one side of the board to the other side, thus to prevent the PCB acting as a total block to air flow from one side to the other and to improve the general airflow through the main housing. However, the method of GB 2 382 932 does not address the problem of the improvement of cooling of individual PCBs or individual components on such PCBs.